For some reason descendant languages don't supply enough data for full reconstruction of proto languages. It is easy to see why this would be the case when trying to reconstruct a very ancient proto language. So much time has passed and the trail has gone cold. But during the 19th and 20th centuries classical philologists achieved alot in trying to reconstruct Indo-European. I'm not sure that they managed to reconstruct whole sentences, but they were successful in forming phrases like" WEKWOM TEKS" which means a weaver of words or a poet. Reconstructing words in IE does require a decision to favor the information supplied by one descendant branch over another. For instance, the Latin word for "oak tree" in Latin is "quercus", but in Russian and other Slavic languages it is "dub". There is no middle ground here. A philologist would regard the Latin word as the more likely clue to the IE word for "oak tree". This would probably be justified by saying that the Latin word is similar to words of the same meaning in other descendant families. The majority rules. The Russian word is useless as a clue in this instance. If you specialize in Romance languages you may know whether or not Proto-Romance has been reconstructed. It seems to me that there is much to work with. At least, you could use Spanish, Portugese, and Catalan to reconstruct Proto-Iberian. Maybe nobody has felt like making the effort to do this. I have read somewhere that Slovak is a good model for reconstructing Common Slavic. But figuring out the proto form of the East Slavic branch is hard enough by itself. Belarusian and Ukrainian seem to have more in common. Great Russian seems somewhat isolated. I don't know what kind of progress has been made in reconstructing Proto-Semitic, Proto-Turkic, and Proto-Sino-Tibetan. Linguists seem to interested in other areas these days.

Slavic languages went through a rapid change in the middle of the 1st millennium BC. Such a change that almost nothing of it can be reconstructed before this period. This change is due to fast spreading of the language (not the people who spoke, a reason why Slavic speaking people are genetically quite different from each other) in the post-Roman, post-Hun and post-Gothic period in Central and South Eastern Europe. The South Slavic and Western Slavic branches are older, it is not known which one is the older one nor from which the spread of a single Slavic form began, form today usually called Common Slavic or Proto-Slavic. This form spread east forming Eastern Slavic (archaeologically speaking almost all East Slavic settlements are not older than the 8-9th c. AD) which appeared in two forms: Novgorodian in the north, which had more West like characteristics (Novgorod was most likely founded by Baltic Slavic merchants who sailed down the lake Ilmen from the Baltic sea) and Kiev in the south, which is more closer to South Slavic (Slavic merchants or colonists who sailed up the stream of Dnestr from the Black Sea coast). The merging of these two gave birth to Eastern Slavic. The original homeland of the Common Slavic language is placed in and around the Pannonian plain, but this could actually point that Common Slavic, which was the Lingua Franca of the Avar Khaganate, is merely the Slavic language the Avars used (the Avar elite spoke a different unidentified language) and then spread into other areas either by Avar expansion or its prestige status.

This is just one theory but from my knowledge of history it makes relative sense. It is one of the subjects I will explore and write about in my academic future.

I have been under the impression that Tatar or Mongolian conquest was responsible for splitting East Slavic. This was because Kiev, the first Russian civilization, had no natural defenses against invading cavalry. Moscow, however, had more ability to defend itself and was cut off from Kievan civilization, resulting in linguistic divergance. Separately, how was the Avar Khaganate distinct from the Khazar Khaganate?

Slavic languages separated regardless whether due to external or internal factors. Some separated despite being of same religion and culture (like West Slavic languages) and some remained unified despite different religion, culture and political history (like Serbo-Croatian).

In both Avar and Khazar Khaganate a Slavic language was the Lingua Franca, not the same language though since the two states were both geographically and temporally apart. Under the Avars the Common Slavic language spread into different parts of Central and South-Eastern Europe, while from these it spread into Eastern Europe. The Eastern southern dialect (that of Kiev) became then the Lingua Franca of the Khazars. The reason why a Slavic language was the LF in both cases is because of in the case of Avars it was the language of the lower class, the farmers and labourers, and the merchants and the majority of soldiers that fought under or together with the Avar elite.

In the case of Khazars, in which realm Slavic was relatively new language, it was the language of the merchants and soldiers, and it fast became the language of the labourers who colonized large empty lands of Eastern Europe, pushing the indigenous Uralic peoples deeper into the forests or mixing with them. The Khazars and other steppe people were not farmers and usually plundered them, but now Slavic colonists were under Khazar protection so they were more or less left alone. This was in the beginning, but in couple of hundred years the situation changed with the arrival of the Mongols.

The language of the Avars is not known and the Khazar language is only assumed to be Turkic.

I picked up the used edition for 3.99 about a month ago or so. Really good book.I would recommend it if you are interested in reconstructed PIE. It deals with thecultural as well as lingustic aspects of the people who spoke PIE. It also talks aboutthe differences between some terms for flora & fauna in the different descendant families.Such as the one for Oak mentioned above.